1. Field of the Invention
The present invention relates to image projection systems. More specifically, the present invention relates to projection systems utilizing light valves for generation of an image beam.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Certain full-color projector systems used in real-time imaging applications include a projector apparatus for generating three separate primary color image beams. The beams converge within the projector at the back plane of a wide angle projection lens, which then projects the resulting composite image onto an external projection screen. Each primary color image is generally produced by an optical light valve driven by a cathode ray tube, and is forwarded to the wide angle projection lens via an optical lens relay system.
In order that the composite image displayed on the external screen be suitable for viewing, it is necessary that each of the individual primary color images be in registration (optical alignment). In conventional projector systems, a set of three position sensitive photodetectors is mounted on or near the external screen for the purpose of detecting the position on the screen of each projected primary color image. The projector apparatus illuminates each of the photodetectors with a target image which is substantially optically coincident with one of the primary color images. The screen position of each primary color image can then be adjusted in response to control signals from the photodetectors which indicate the screen position of the target image associated therewith.
Unfortunately, conventional image registration systems using screen mounted detectors suffer from a number of drawbacks. For example, installation of such a system requires precise alignment of the screen detectors relative to the projector apparatus. This requirement often compels the utilization of a precision alignment mechanism which may increase manufacturing costs. Moreover, the connection cables which link the screen detectors with the projector apparatus may hinder the implementation of the projection system in certain applications, and may also increase production costs.
In addition, the magnification of the target images incident on the photodetectors varies with the size of the projection screen. Accordingly, more than one set of photodetectors may need to be employed to cover the complete range of projected image sizes. Moreover, when ambient light levels remain unchanged in the face of target image intensity changes due to modifications in screen size, the signal to noise ratio of the target image illuminating each photodetector is altered. This change in signal to noise ratio complicates the processing of the control signals generated by the photodetectors.
A perhaps more serious concern, however, arises in connection with the application of color projector systems in ships, planes or other moving vehicles. In these rugged environments there may be flexing between the surface on which the projector apparatus is mounted and the surface of the external projection screen. As the projector moves relative to the screen the photodetectors on the screen sense a misregistration of the projected images, and issue control signals instructing the projector to center each primary color image on its associated detector. However, this temporary screen flexing has given rise to only an "apparent" misregistration. That is, the relative motion between the projector and the screen does not cause a misregistration of the primary color images at the back plane of the wide angle projection lens within the projector. It follows that the control signals emitted by the photodetectors subsequent to relative motion between the projector and screen may induce a "real" misregistration of the primary color images at the wide angle projection lens. The resulting degradation of the composite image displayed on the screen will then depend on the size and frequency of the perturbations in the relative displacement of the projector and screen.
Accordingly, a need in the art exists for a projection system having an image registration system which is relatively unaffected by temporary variations in the relative displacement between a projector apparatus and external screen.